I am from Greece which doesn't have a big tv-filmaking industry. I want to work as a freelancer in films and TV and I was wondering if I could do it in projects around Europe, without living permanently in another country. Is it realistic? And if it is, how can it happen?

In News, Sports or Fashion, sounds probable. Definitely need to pick a specialty and build a reel. I know fashion photogs/videographers for example that jet set between miami, paris, and other. Something in sports could def work. Football (soccer) is huge. Or I'm sure there are news jobs in your area.

I am from Greece which doesn't have a big tv-filmaking industry. I want to work as a freelancer in films and TV and I was wondering if I could do it in projects around Europe, without living permanently in another country. Is it realistic? And if it is, how can it happen?

Sure it's realistic. More difficult than being what is called a
“local hire” but it can be done.

I am still in the process of working hard to gain knowledge and I have to know some things from people who know!

Do free work for those with zero budget projects to gain basic experience. Then work with other people who know more than you do for more in-depth knowledge. You will not be a successful freelancer until you have fairly substantial work experience behind you.

Maybe you can research what countries outside Greece have a lot of companies that do your kind of work. Be open to working on commercials too. It would be great if you had a friend or relative who lives near those companies & will let you stay with them.